The son of Pink Floyd's David Gilmour was arrested soon after the December 9 clashes on suspicion of violent disorder and attempted criminal damage, before being released on bail.
He was photographed hanging from a Union Flag on the Cenotaph during the protests, and was later accused of throwing a bin at a convoy of cars containing Prince Charles and smashing a window.
Gilmour told Kingston Crown Court that he had taken LSD and valium before the incident, after reportedly being rejected by his biological father, the writer Heathcote Williams.
The judge watched various clips of Gilmour's action during the day of protests, including him shouting: "Let them eat cake, let them eat cake, they say. We won't eat cake, we will eat fire, ice and destruction, because we are angry, very f**king angry."
The Cambridge University student had entered a non-specific guilty plea in May